Mushroom Heads for Spring Return Pushbuttons, ZB4-B ZB4-B series mushroom heads. Note Contact blocks must be ordered seperately (see stock no. 331-0158 ). Stock nos. 330-9013 , 330-9029 , 330-9035 and 342-2627 also require a fixing collar (see stock no. 331-0142 ). For contact blocks that include a collar, please see stock no. 331-0192 and range. Schneider Electric ZB4-B & XB4-B Harmony® 22mm Range